The present invention relates to an emulsion polymerization process using a reactor having:
one or more circulation loops with one or more inlets for raw material, one or more inlets for a carrier liquid in which the polymer to be formed is insoluble, and one or more outlets for polymer emulsion;
one or more driving means for continuously circulating the reactor charge within the circulation loop,
xe2x80x83the process comprising the steps of:
charging the reactor with raw material, including monomers, and with a carrier liquid, including stabilizers, and optionally other additives;
then continuously feeding the loop reactor with the raw material and with the carrier liquid;
and continuously withdrawing part of the reactor charge from the reactor at a rate equal to the rate of total raw material and carrier liquid feeding.
The invention also relates to a reactor for such a process.
Generally, the carrier liquid is aqueous and is designated as the xe2x80x9cwater phase.xe2x80x9d
An emulsion polymerization process and a reactor therefor are described in European patent application EP-A 0 145 325. In the process as disclosed in this publication polymer emulsions are manufactured by the addition polymerization of olefinically unsaturated monomers, in the presence of water and stabilizers. The polymer content of the formed polymer emulsion usually is of the order of 50-65% by weight. After leaving the reactor, the polymer emulsion is matured and cooled down in a maturing vessel or cooling tank. These polymer emulsions are also known as dispersions, latexes or lattices, and are widely used in industrial applications, for instance as binders for paints, adhesives or printing inks.
Emulsion polymerization in closed loop reactors has the advantage that a wide range of process parameters, such as pressure or temperature, can be used. Consequently, loop reactors can be used to produce different polymer emulsions. In order to prevent contamination when changing from one grade to another and to prevent skinning through the drying of residues which adhere to the inner sides of the reactor or associated equipment such as cooling tanks, the reactor and associated equipment are often washed after the completion of a polymerization process. To prevent fouling, which would reduce the heat transfer and impair the efficiency of cooling, the cooling tanks and associated equipment may have to be rinsed, even if the same polymer emulsion is produced in the next production course. The loop reactor and associated equipment are washed with a washing medium, e.g., water. This creates large volumes of dilute aqueous latex, also known as whitewater. Though generally only about 0.2% by weight of the total batch needs to be washed from the cooling tanks, the quantity of contaminated washings may be as much as 6-10% of the weight of produced latex, as a result of dilution, especially when other sources of washings are considered. Such sources may be washings from sieves, storage tanks, pumps, and tanker loading hose, all of which represent an economic loss as well as the creation of contaminated water which must be disposed of safely. Generally, the content of polymerized particles in these washings is between 0.5-3 wt. %, though it may be higher or lower. Such particles are for example styrenics, (meth)acrylics, acrylonitrile polymers, vinyl polymers, such as vinylacetate homopolymers and copolymers with (meth)acrylates, xcex1-olefins, ethylene, higher vinyl esters, polyvinylchloride, or hybrids or mixtures thereof. The washings may also contain stabilizing colloids, surfactants, and other salts and additives. The loss of these washings is economically disadvantageous and causes environmental problems.
Another potential problem attending emulsion polymerization in loop reactors is the fluctuation or cycling of the formation of new particles in the start-up stages of the polymerization process, which may occur when the reactor is pre-filled with water phase. In emulsion polymerization processes, including the loop process, stabilizer adheres to the polymer particles formed, so the concentration of free stabilizer in the water phase is reduced. The size of the new particles depends, inter alia, on the stabilizer concentration, the molecular weight and the chemical nature of the stabilizers, and the polarity and the functionality of the monomer or monomer mixture being polymerized. In practice, the size and the number of new particles depend also on physical factors, e.g., temperature and agitation. The number and the diameter of the new particles control the total polymer surface area formed, which may adsorb varying amounts of the original stabilizers. If the concentration of free stabilizer is too low, no new particles are formed. Existing particles are eluted continuously as new raw material is added. The total particle surface area is reduced in the absence of new particles despite the growth of individual particles. Meanwhile, the stabilizer content is approximately constant, the addition of the stabilizer taking place at the same rate as its overflow from the reactor. As a consequence, the reduction of total particle surface area results in the remaining surfaces becoming saturated with stabilizer, when the free stabilizer concentration will rise to the point where new crops of fine particles can be formed again. These cycles, in a well-designed and stable formulation, fade away to give an equilibrium of product overflow balanced by particle creation in a saturated stabilizer environment.
Hence, the object of the invention is to minimize the loss of polymer via washings from the reactor or associated equipment such as cooling tanks and to reduce the economic loss and environmental risk caused by washings disposal. Another object of the invention is to reduce the effects of cycling of the particle size and especially the particle surface area during the early part of the reaction run, to improve process and product stability prior to the establishment of equilibrium within the reactor.
The object of the invention is achieved by an emulsion polymerization process using a reactor as described in the opening paragraph, wherein the carrier liquid is wholly or partially made up of washings used for washing one or more parts of the same or a different polymerization reactor and/or associated equipment. Such washings are contaminated with compatible material in solution or suspension. The washings may comprise a polymer emulsion of the same general composition to be manufactured by the reactor or a compatible emulsion polymer of a different composition.